Usually agricultural tractors contain in their rear region a three-point attachment arrangement consisting of two lower steering arms at the sides and a central upper steering arm. An operating implement can be coupled to the steering arms. They can be repositioned in the vertical direction by hydraulic cylinders, in order to raise or lower the operating implement. A free deflection to the side is possible within certain limits that may be limited or prevented by side stabilizers. The ability to move to the side is particularly important during plowing, since the plow should not be fastened rigidly to the tractor in the horizontal direction. Rather the plow should be uncoupled from the steering movements of the tractor by a free pendulous movement to the side of the implement attachment interface. Here the horizontal guidance of the plow is the result of the convergence of the lower steering arms, whose lines of action intersect in a guide point (August Van der Beek: “Die Lage des ideellen Fuehrungspunktes und der Zugkraftbedarf beim Pfluegen” (“The Position of the Ideal Guide Point and the Towing Force Required During Plowing”), “Grundlagen der Landtechnik” (Fundamentals of Agricultural Technology (1983) Number 1, Page 10 to 13). On the other hand the plow is connected rigidly with the tractor in the vertical direction or can be repositioned to pre-determined values with the use of a control arrangement, in order to maintain a desired depth of furrow.
DE-A-199 51 840 describes an attachment interface for the coupling of operating implements to a utility vehicle that is provided with a coupling frame and six hydraulic cylinders that can be repositioned in length. Each of the first ends of the hydraulic cylinders are connected in joints to the utility vehicle and each of the second ends are connected in joints to the coupling frame. The hydraulic cylinders are arranged in closed kinematic linkages, in particular according to the manner of a hexapod, in which the connecting elements engage in the corners of a frame side triangle or a vehicle side triangle. This arrangement of the connecting elements makes possible movements of the coupling frame and the operating implement in six degrees of freedom. In order to reposition the operating implement relative to the utility vehicle the lengths of the hydraulic cylinders can be adjusted largely independent of each other. Here the relationship of the lengths to each other can be adjusted as desired by means of an appropriate control arrangement. With this hexapod arrangement a free sideways movement with a fixed vertical position is not directly possible, since with all hydraulic cylinders switched to a free condition the repositioning to a fixed, pre-determined vertical position is not possible. If, on the other hand, only the two upper and the two lower hydraulic cylinders are switched to a free condition and the two hydraulic cylinders at the sides are switched to a rigid condition, then the plow can rotate during the operation about an axis extending in the longitudinal direction of the vehicle as a result of forces applied to it, which considerably impairs the result of the operation.